There is an increasing need for high purity hydrogen in the chemical process industries including steel annealing, silicon manufacturing, hydrogenation of fats and oils, glass making, hydrocracking, methanol production, the production of oxo alcohols, and isomerization processes. There are a variety of known processes wherein hydrogen is produced, including steam reforming of natural gas or naptha. In this process a feedstock such as natural gas is compressed and fed to a purification unit to remove sulfur compounds. The desulfurized feed is then mixed with superheated steam and fed to a reformer to produce primarily H2 and CO. The effluent stream from the reformer is sent to a heat recovery unit, then to a shift converter to obtain additional H2. The effluent from the shift converter passes through a process cooling and recovery unit prior to sending the effluent to a PSA system wherein high purity (e.g. 99.9 mol. % or greater) hydrogen is produced.
The H2 gas being fed to PSA systems can, however, contain several contaminants in widely varying concentrations, (e.g. the feed stream to the PSA from a steam methane reformer (SMR) may contain one or more of CO2, CH4, CO and N2. This combination of adsorbates at such widely varying compositions presents a significant challenge to the design of a PSA system, particularly with respect to adsorbent selection and configuration of the adsorber/adsorbent bed.
Representative prior art PSA processes include Sircar et al., U.S. Pat. No. 4,077,779; Fuderer et al., U.S. Pat. No. 4,553,981; Fong et al., U.S. Pat No. 5,152,975; Wagner, U.S. Pat No. 3,430,418 and Batta, U.S. Pat. No. 3,564,816.
More specifically, Bomard et al. in U.S. Pat. No. 5,912,422 discloses a PSA process for the separation of hydrogen from a feed gas mixture that contains CO and other impurities such as CO2 and hydrocarbons. The feed mixture is passed into a first adsorbent to remove CO2 and/or hydrocarbons, and then into a second adsorbent that is a faujasite type zeolite with at least 80% lithium exchange to remove primarily CO impurity to produce hydrogen. If N2 is present in the hydrogen-containing feed mixture then Bomard et al., introduces a third adsorbent between the first and second adsorbent to remove nitrogen.
Golden et al., U.S. Pat. No. 4,957,514 disclosed the purification of hydrogen using barium exchanged Type X zeolite.
Golden at al., U.S. Pat. No. 6,027,549, disclose a PSA process to remove CO2 and CH4 using activated carbons having bulk densities in the range of approximately 35-38 lb/ft3.
Johnson et al., in U.S. Pat. No. 6,302,943 and in EP 1097746A2, disclose adsorbents for H2 recovery by pressure and vacuum swing adsorption, wherein the adsorbents at the product end of the bed have Henry's Law constants between 0.8 and 2.2 mmol/g/atm for CO and between 0.55 to 1.40 mmol/g/atm N2.
There remains a need for improved PSA system and process having lower adsorbent requirements and higher product recovery as compared to existing PSA systems and processes for hydrogen production.
The present invention addresses this need through the use of a novel selection and arrangement of adsorbents within the adsorbent bed.